The present invention relates generally to reducing discrete tonal noise from aircraft engines, and more particularly to an active noise control subassembly capable of generating a canceling noise to offset such engine noise.
Discrete tonal noise may have one or more discrete frequencies. Aircraft engines produce discrete tonal noise, especially from fans and turbines. Such noise from larger aircraft engines, presently under development, may pose a problem in the vicinity of airports during aircraft take off and landing operations. Also, future aircraft noise regulations may pose a problem for existing aircraft engines.
Known passive noise control techniques for reducing aircraft engine discrete tonal noise include noise absorbing liners and tuned resonators usually mounted at the engine inlet and outlet to reduce the level of discrete tones radiated outside the engine. However, the effectiveness of passive noise control treatment would be greatly reduced for engines with large fan diameters because of the lower fan blade passage frequency.
Known active noise control techniques for reducing aircraft engine discrete tonal noise generate a canceling noise forward and aft of the fan. The frequency of the canceling noise is equal to the blade passage frequency (and/or multiples thereof) as determined from engine speed using a tachometer. The amplitude and phase of the canceling noise is determined by a computer using feedback and/or feedforward control techniques with sound inputs from a microphone array disposed in the vicinity of the fan and the canceling noise such that the canceling noise is generally equal in amplitude and opposite in phase to the engine's discrete tonal noise. Conventional techniques for generating the canceling noise include using piezoceramic actuator plates to bendably vibrate a panel to produce the canceling noise. The panel may be a part of the aircraft engine, such as a part of the fan shroud, or the panel may be a member which is separate from, but attached to, the aircraft engine. The piezoceramic plate is driven by an electric AC signal such that when the signal is positive, the plate causes the panel to bendably deflect in a first direction from its resting state, and when the signal is negative, the plate causes the panel to bendably deflect in the opposite direction.
The larger the amplitude of the electric AC signal driving the piezoceramic actuator plate, the larger the bending vibration, and hence the louder the canceling noise, produced by the attached panel. However, the piezoceramic actuator plate will structurally fail when the applied electric AC signal causes the plate to exceed its critical tensile stress (which is smaller than its critical compressive stress). It is noted that the particular value of the critical tensile (or compressive) stress depends on the particular piezoceramic material being used. What is needed is an improved subassembly, of an active noise control system, for generating a loud canceling noise to reduce discrete tonal noise produced by an aircraft engine.